Profile measurements in the plasma edge of MAST using a ball pen probe

TL;DR

This paper demonstrates the successful use of the ball pen probe to measure plasma potential, electron temperature, and electric fields in MAST, showing good agreement with Thomson scattering and providing insights into plasma rotation.

Contribution

The study introduces the application of the ball pen probe for detailed profile measurements in the plasma edge of MAST, including potential, temperature, and electric fields, with validation against Thomson scattering.

Findings

BPP potential differs significantly from floating potential in polarity and shape.

Electron temperature measured by BPP agrees well with Thomson scattering when secondary electron emission is considered.

Radial electric field estimated from BPP profiles is around 1 kV/m and increases with plasma current.

Abstract

The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature and radial electric field on the Mega Amp Spherical Tokamak (MAST). The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile an estimate of the radial electric field is extracted which is shown to be of the order ~1kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from ExB motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.